Development of a Wireless High-Frequency Microarray Implant for Retinal Stimulation - Artificial Sight: Basic Research, Biomedical Engineering, and Clinical Advances - page 182

Figure 9.13. Loading effect on current.
Biphasic Current Controller (BCC)
As mentioned earlier, the width and interphase delay of biphasic current
waveform depend on the degree of a patient's retinal damage. This will be
ascertained by the doctor through clinical experiments on each patient. To
generate biphasic current waveform of variable width and interphase delay,
we have designed BCC (Figure 9.14). It is designed to control the widths of
anodic and cathodic pulses and interphase delay independently. The first block
of BCC controls P, the second ensures interphase delay, and the third controls
N . Each block is composed of three T Flip-Flops (TFF) connected as three-
stage synchronous binary counter with three parallel inputs P0, P1, and P2.
By programming 9 bits of the three blocks, the variable widths and interface
delay are generated. When RST is asserted low, each block is loaded with P0,
P1, and P2. Binary counter can count up to 7 from 0 or any number programmed
by P0, P1, and P2. On reaching count 7, RC becomes high, which is fed back
to the input of a1 after inversion blocking the toggle of TFFs and locking the
counter state. On reset P is set high and N low. After the set time of anodic
width, P is reset triggering interphase delay. On expiry of interphase delay, N is
set high enabling cathodic pulse, which is reset after the set period of time of
cathodic width. The beauty of the circuit is that we can generate only anodic or
cathodic pulse by setting the other to zero or biphasic pulse without delay by
setting interphase delay to zero. For longer width or delay the counter should be
set to lower value. This is because the width or delay is the difference between
the set value and 7.